In fm demodulators, the intelligence to be recovered is not in amplitude variations; it
is in the variation of the instantaneous frequency of the carrier, either above or
below the center frequency. The detecting device must be constructed so that its output
amplitude will vary linearly according to the instantaneous frequency of the incoming
signal.

Several types of fm detectors have been developed and are in use, but in this section
you will study three of the most common: (1) the phase-shift detector, (2) the ratio
detector, and (3) the gated-beam detector.

To be able to understand the principles of operation for fm detectors, you need to
first study the simplest form of frequency-modulation detector, the SLOPE DETECTOR. The
slope detector is essentially a tank circuit which is tuned to a frequency either slightly
above or below the fm carrier frequency. View (A) of figure 3-9 is a plot of voltage
versus frequency for a tank circuit. The resonant frequency of the tank is the frequency
at point 4. Components are selected so that the resonant frequency is higher than the
frequency of the fm carrier signal at point 2. The entire frequency deviation for the fm
signal falls on the lower slope of the bandpass curve between points 1 and 3. As the fm
signal is applied to the tank circuit in view (B), the output amplitude of the signal
varies as its frequency swings closer to, or further from, the resonant frequency of the
tank. Frequency variations will still be present in this waveform, but it will also
develop amplitude variations, as shown in view (B). This is because of the response of the
tank circuit as it varies with the input frequency. This signal is then applied to the
diode detector in view (C) and the detected waveform is the output. This circuit has the
major disadvantage that any amplitude variations in the rf waveform will pass through the
tank circuit and be detected. This disadvantage can be eliminated by placing a limiter
circuit before the tank input. (Limiter circuits were discussed in NEETS, Module 9, Introduction
to Wave-Generation and Wave-Shaping Circuits.) This circuit is basically the same as
an AM detector with the tank tuned to a higher or lower frequency than the received
carrier.

Figure 3-9A. - Slope detector. VOLTAGE VERSUS FREQUENCY PLOT

Figure 3-9B. - Slope detector. TANK CIRCUIT

Figure 3-9C. - Slope detector. DIODE DETECTOR

Q.21 What is the simplest form of fm detector?
Q.22 What is the function of an fm detector?

FOSTER-SEELEY DISCRIMINATOR

The FOSTER-SEELEY DISCRIMINATOR is also known as the PHASE-SHIFT DISCRIMINATOR. It uses
a double-tuned rf transformer to convert frequency variations in the received fm signal to
amplitude variations. These amplitude variations are then rectified and filtered to
provide a dc output voltage. This voltage varies in both amplitude and polarity as the
input signal varies in frequency. A typical discriminator response curve is shown in
figure 3-10. The output voltage is 0 when the input frequency is equal to the carrier
frequency (fr). When the input frequency rises above the center frequency, the
output increases in the positive direction. When the input frequency drops below the
center frequency, the output increases in the negative direction.

Figure 3-10. - Discriminator response curve.

The output of the Foster-Seeley discriminator is affected not only by the input
frequency, but also to a certain extent by the input amplitude. Therefore, using limiter
stages before the detector is necessary.

Circuit Operation of a Foster-Seeley Discriminator

View (A) of figure 3-11 shows a typical Foster-Seeley discriminator. The collector
circuit of the preceding limiter/amplifier circuit (Q1) is shown. The limiter/amplifier
circuit is a special amplifier circuit which limits the amplitude of the signal. This
limiting keeps interfering noise low by removing excessive amplitude variations from
signals. The collector circuit tank consists of C1 and L1. C2 and L2 form the secondary
tank circuit. Both tank circuits are tuned to the center frequency of the incoming fm
signal. Choke L3 is the dc return path for diode rectifiers CR1 and CR2. R1 and R2 are not
always necessary but are usually used when the back (reverse bias) resistance of the two
diodes is different. Resistors R3 and R4 are the load resistors and are bypassed by C3 and
C4 to remove rf. C5 is the output coupling capacitor.

CIRCUIT OPERATION AT RESONANCE. - The operation of the Foster-Seeley discriminator
can best be explained using vector diagrams [figure 3-11, view (B)] that show phase
relationships between the voltages and currents in the circuit. Let's look at the phase
relationships when the input frequency is equal to the center frequency of the
resonant tank circuit.

The input signal applied to the primary tank circuit is shown as vector ep.
Since coupling capacitor C8 has negligible reactance at the input frequency, rf choke L3
is effectively in parallel with the primary tank circuit. Also, because L3 is effectively
in parallel with the primary tank circuit, input voltage ep also appears across
L3. With voltage ep applied to the primary of T1, a voltage is induced in the
secondary which causes current to flow in the secondary tank circuit. When the input
frequency is equal to the center frequency, the tank is at resonance and acts resistive.
Current and voltage are in phase in a resistance circuit, as shown by is and ep.
The current flowing in the tank causes voltage drops across each half of the balanced
secondary winding of transformer T1. These voltage drops are of equal amplitude and
opposite polarity with respect to the center tap of the winding. Because the winding is
inductive, the voltage across it is 90 degrees out of phase with the current through it.
Because of the center-tap arrangement, the voltages at each end of the secondary winding
of T1 are 180 degrees out of phase and are shown as e1 and e2 on the
vector diagram.

The voltage applied to the anode of CR1 is the vector sum of voltages ep and
e1, shown as e3 on the diagram. Likewise, the voltage applied to the
anode of CR2 is the vector sum of voltages ep and e2, shown as e4
on the diagram. At resonance e3 and e4 are equal, as shown by
vectors of the same length. Equal anode voltages on diodes CR1 and CR2 produce equal
currents and, with equal load resistors, equal and opposite voltages will be developed
across R3 and R4. The output is taken across R3 and R4 and will be 0 at resonance since
these voltages are equal and of appositive polarity.

The diodes conduct on opposite half cycles of the input waveform and produce a series
of dc pulses at the rf rate. This rf ripple is filtered out by capacitors C3 and C4.

OPERATION ABOVE RESONANCE. - A phase shift occurs when an input frequency higher
than the center frequency is applied to the discriminator circuit and the current and
voltage phase relationships change. When a series-tuned circuit operates at a frequency
above resonance, the inductive reactance of the coil increases and the capacitive
reactance of the capacitor decreases. Above resonance the tank circuit acts like an
inductor. Secondary current lags the primary tank voltage, ep. Notice that
secondary voltages e1 and e2 are still 180 degrees out of phase with
the current (iS) that produces them. The change to a lagging secondary current
rotates the vectors in a clockwise direction. This causes el to become more in phase with
ep while e2 is shifted further out of phase with ep. The
vector sum of ep and e2 is less than that of ep and e1.
Above the center frequency, diode CR1 conducts more than diode CR2. Because of this
heavier conduction, the voltage developed across R3 is greater than the voltage developed
across R4; the output voltage is positive.

OPERATION BELOW RESONANCE. - When the input frequency is lower than the center
frequency, the current and voltage phase relationships change. When the tuned circuit
is operated at a frequency lower than resonance, the capacitive reactance increases and
the inductive reactance decreases. Below resonance the tank acts like a capacitor and the
secondary current leads primary tank voltage ep. This change to a leading
secondary current rotates the vectors in a counterclockwise direction. From the
vector diagram you should see that e2 is brought nearer in phase with ep,
while el is shifted further out of phase with ep. The vector sum of ep and
e2 is larger than that of e p and e1. Diode CR2 conducts
more than diode CR1 below the center frequency. The voltage drop across R4 is larger than
that across R3 and the output across both is negative.

Disadvantages

These voltage outputs can be plotted to show the response curve of the discriminator
discussed earlier (figure 3-10). When weak AM signals (too small in amplitude to reach the
circuit limiting level) pass through the limiter stages, they can appear in the output.
These unwanted amplitude variations will cause primary voltage ep [view (A) of
figure 3-11] to fluctuate with the modulation and to induce a similar voltage in the
secondary of T1. Since the diodes are connected as half-wave rectifiers, these small AM
signals will be detected as they would be in a diode detector and will appear in the
output. This unwanted AM interference is cancelled out in the ratio detector (to be
studied next in this chapter) and is the main disadvantage of the Foster-Seeley circuit.

Q.23 What type of tank circuit is used in the Foster-Seeley discriminator?
Q.24 What is the purpose of CR1 and CR2 in the Foster-Seeley discriminator?
Q.25 What type of impedance does the tank circuit have above resonance?